An optical fiber is broadly used for long-distance rapid transmission owing to its low transmission loss and great bandwidth. For installation of the optical fiber conventionally, several optical fibers are bound or twisted to be a cable, and then this cable is installed. In recent, the optical fibers are generally installed by blown air.
According to the air blown installation, a tube made of polymer material having a diameter of 5 to 8 mm, called a micro tube or duct, is buried in an installation spot in advance, and then an optical fiber unit having 1 to 12 cores are installed therein with the use of blown air. The optical fiber installed by blown air (so called, Air Blown Fiber: ABF) is installed using fluid drag force, so it is important that the surface of the optical fiber is configured to receive more fluid drag force.
A technique for processing the outer surface of ABF is disclosed in U.S. Pat. No. 5,042,907 that is schematically shown in FIG. 1. As shown in FIG. 1, glass beads 5 are provided on the outer surface of the optical fiber 1 so that the optical fiber is better affected by the blown air. In other words, the glass beads are stirred together with a coating resin, and then uniformly coated on the outer surface of the optical fiber 1. At this time, the size of the glass bead 5 received in the resin 4 of the outer surface of the optical fiber should be relatively larger than the thickness of the coating layer, and the glass bead 5 should have high Young's modulus in order to generate propulsive force due to the blown air. However, such high Young's modulus of the glass bead deteriorates bend characteristic of the optical fiber unit. In addition, cracks may happen between the glass bead 5 and the resin 4, and these cracks may be propagated inside the optical fiber. In such a reason, an intermediate layer 3 should be interposed between an inner buffer layer 2 and the resin 4 of the outer surface. However, this configuration requires at least three coating processes, thereby complicating the manufacturing procedure and increasing costs.
Another conventional technique for processing the surface of the optical fiber is disclosed in U.S. Pat. No. 5,555,335 shown in FIG. 2. According to this technique, after a resin is coated on an optical fiber 1, glass beads are attached on the outer surface 6 of the optical fiber by means of static electricity before the outer surface is cured. However, some glass beads are apt to be detached from the outer surface since the adhesive force of the glass beads is not regular on the outer surface of the optical fiber. The glass beads not adhered to but detached from the outer surface may damage the optical fiber unit while the unit is installed.
As another conventional technique, a dimple may be formed on the surface of the optical fiber with the use of foaming polymer materials. However, the foaming polymer material increases the coefficient of friction, so an installation length of the optical fiber for a unit work is too short and the hardness of the optical fiber unit is too weak.
On the other hand, it has ever been proposed to install a ribbon-type optical fiber by winding a fiber of a particular material. However, since the ribbon-type optical fiber has a direction to the bending, the optical fiber tends to be bent only to one direction.